Toyota prius Owner's Manual page 358

3-1. Using the air conditioning system and defogger
Sounds when using the Solar Ventilation System

If the amount of sunlight is insufficient, you may hear a sound coming from
the air conditioning system. This is the sound of a control function to pre-
serve the fan and does not indicate a malfunction.
CAUTION
 Solar Ventilation System precautions
 Do not use the system if people are inside the vehicle.
The Solar Ventilation System is not intended to ensure the performance of
the air conditioning system. Do not leave children or pets in the vehicle.
The temperature of the interior will remain high, possibly resulting in heat-
stroke, dehydration, or even death.
 This system should be used only for the relief of discomfort when entering
the vehicle, and should not be used for other purposes.
 Precautions for moon roof with Solar Panel
P. 136
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